The term orthopedic instrument denotes all implants for orthopedic prostheses intended to be implanted in bony tissue in order to carry out replacement of deficient joints, and also ancillary devices used to prepare the receiving bone in order to allow the placement of the definitive implant and, in particular, the surgical rasps used for orthopedic implants. Within the category of orthopedic prostheses, the number of implants available are many. Total hip prostheses, knee prostheses, shoulder prostheses or even elbow, ankle or spine prostheses are notable examples.
One field of application of the present invention is chiefly, although not exclusively, the placement of orthopedic instruments for hip prostheses. The vast majority of the proposed types of hip prosthesis have in common a first part, consisting of a femoral stem intended to be implanted in the medullary cavity of the femur and a prosthetic head formed of a spherical component, possibly removable, which takes the place of the head of the femur, and a second part, consisting of a prosthetic acetabulum, intended to be implanted in the acetabular cavity situated at the lateral face of the iliac bone. The prosthetic acetabulum, which is therefore intended to replace the joint part of the pelvis, is more specifically made up of an acetabular cup, which is an approximately hemispherical component, generally made of metal, implanted in the pelvic bone and into which an insert with which the prosthetic head is articulated is placed.
In particular, the placement of the prosthetic acetabulum in the bony cavity may be performed by simple impaction, using a placement ancillary provided for that purpose, by means of which the prosthetic acetabulum is impacted by the surgeon, using an impacter, typically a hammer, that strikes the placement ancillary. One example of an ancillary for the placement of the prosthetic acetabulum consists of a rigid manipulation handle provided in its distal part with a holding head, that allows the acetabulum to be held and then positioned up to the site of impaction of the acetabulum into the bony cavity of the hip and, in its proximal part, with an impact surface intended to be impacted by means of a striking face of the impacter provided for this purpose so as to apply the impaction force necessary for the placement of the prosthetic acetabulum in the bony cavity.
Surgical success in the placement of the prosthetic acetabulum and, more generally, of an orthopedic prosthesis implant intended to be implanted by impaction, relies on this implant achieving a sufficient degree of insertion into the receiving bone, on which the biomechanical stability will be dependent. This biomechanical stability is in actual fact determined by two distinct phenomena:                the primary stability, which corresponds to the degree of attachment or anchorage of the prosthetic element in the bone immediately after the operation (immediate postoperative stability), and which is dependent on the quality of the bone and of the surgical procedure, which will be detailed later on;        the secondary stability, which corresponds to the bone regrowth through osteointegration phenomena and which is dependent on the primary stability and on the ability of the bone to heal.        
According to the conventional surgical procedure, the surgeon bores into the bone of the pelvis to create a hemispherical cavity that can retain the implant, while at the same time stimulating bone regrowth phenomena which will make it possible to improve osteointegration during the healing phase. Once the cavity has been produced, the surgeon inserts the prosthetic acetabulum forcefully into the cavity by impaction using the impacter and the placement ancillary until he considers that the implant has reached a sufficient degree of insertion that good primary stability of the implant is achieved, but without causing the bone to fracture. The primary stability is indeed an essential element in the success of the surgery. Typically, good primary stability corresponds to a placement situation in which the degree of insertion of the implant is optimal, this being manifested in a maximum area of contact between the bone and the implant. However, it is difficult these days for the surgeon to be able to quantify in fine detail the number and force of the impacts that need to be generated on the placement ancillary using the impacter in order to obtain good primary stability without thereby damaging the receiving bone. Therefore a compromise needs to be reached between impact energy that is high enough to obtain good primary stability, which corresponds to a maximum area of contact between the bone and the implant, and impact energy low enough not to risk preoperative fracturing of the receiving bone.
Now, these days, only empirical methods are used. Typically, during the operation, the surgeons rely on their own experience and, in particular, on the noise of the impact in order to determine whether they need to hit harder or less hard and/or whether they need to continue to apply hammer blows. However, by using this type of method, the surgeons are not able to have absolute certainty that the degree of insertion of the implant into the receiving bone is optimal. Furthermore, one of the disadvantages inherent to the application of this method is that when the implant is insufficiently inserted into the bone, the latter may then become detached from its support. That may result in significant lesions in the cavity bored into the receiving bone. When that happens, the surgeon will therefore have for example to operate again in order to enlarge the bony cavity, something which will then allow a new implant of suitable dimensions to be positioned.
Patent document EP1433445 discloses a method for preoperatively measuring the mechanical stability of an orthopedic prosthesis which is intended to be implanted in a bone by forced insertion, which method is intended to assist the surgeon in estimating the stability of the implant. This method consists in measuring a relative slippage of the prosthesis with respect to the receiving bone under the action of a suitably predetermined test load applied to the prosthesis or to the receiving bone. However, this method relies on a complex procedure.
Moreover, this identified need to be able to evaluate in fine detail the number of impacts required in order to achieve a sufficient degree of insertion without the risk of causing preoperative fractures of the receiving bone is also encountered when implementing the surgical procedure that involves impacting an orthopedic implant surgical rasp, particularly a femoral rasp for a hip prosthesis. What happens is that this type of rasp is conventionally used in order to make a hole in the medullary cavity of the femur by “milling” the bone so as, on the one hand, to prepare the shape of the cavity so that the implant is a perfect fit therein and, on the other hand, to stimulate bone regrowth by removing superficial bony tissue. These orthopedic rasps are not an element of the implant (such as the acetabulum or the femoral stem) because they are removed after they have been impacted, so as to allow the definitive implant to be placed. However, the placement principle is similar insofar as it involves directly impacting the rasp with the impacter (without the intermediary of an ancillary), until a sufficient degree of insertion is achieved, taking care not to cause the receiving bone in which the rasp is inserted to fracture.